Why Silica Is Used in Concrete?
Role of Silica in Concrete Production
Silica is one of the main natural materials used in concrete, mainly found in sand and aggregates. During cement hydration, silica reacts with calcium compounds to form calcium silicate hydrate (C-S-H). This is the key binding material that gives concrete its strength and solid structure. Without silica, concrete would lack proper bonding and would remain weak and unstable. It also helps fill small voids between particles, making the mix more compact and uniform.
Importance of Silica for Strength and Durability
Silica plays a major role in improving the strength of concrete. It creates a dense internal structure that allows concrete to carry heavy loads without failure. This improved density also increases durability by reducing the movement of water and harmful chemicals inside the material. As a result, concrete becomes more resistant to weather changes, corrosion, and long-term wear. This is why silica-rich mixes are widely used in infrastructure like bridges, highways, and marine structures.
How Silica Improves Concrete Properties?
Silica improves both fresh and hardened concrete performance. In fresh concrete, it helps maintain a uniform mix and reduces material separation. In hardened concrete, it lowers porosity, which means fewer pathways for water and chemicals to enter. This improves resistance against steel reinforcement corrosion and surface damage. It also strengthens the bond between cement paste and aggregates, which increases overall stability. In advanced concrete mixes, fine silica particles make the structure even denser, resulting in higher strength, better durability, and improved crack resistance over time.
Types of Silica Used in Concrete
Crystalline Silica in Sand and Natural Aggregates
Crystalline silica is the most common form of silica found in nature. It has a fixed and ordered crystal structure. In concrete work, it is mainly present in sand, gravel, and crushed stone used as aggregates. This type of silica is very hard and provides basic strength and stability to concrete. However, it can react with cement alkalis under certain conditions, which may lead to durability issues if the materials are not selected properly. Because of this, the quality and reactivity of aggregates are carefully tested before use in major construction projects.
Amorphous Silica Properties and Role in Concrete
Amorphous silica does not have a fixed crystal structure. It is more reactive compared to crystalline silica and is often used in modern concrete mixes to improve performance. It is commonly found in industrial by-products like silica fume and fly ash. The main property of amorphous silica is its high chemical reactivity, which allows it to react with calcium hydroxide in cement and form additional binding compounds. This reaction helps make concrete stronger and more durable. The main difference between amorphous and crystalline silica is their structure and reactivity, with amorphous silica being more useful for improving concrete quality.
Silica Fume for High-Strength Concrete Applications
Silica fume is an ultra-fine powder produced during the manufacturing of silicon metal or ferrosilicon alloys. It consists mostly of amorphous silica particles that are extremely small, even finer than cement. Because of its size and high reactivity, silica fume is widely used in high-performance concrete. It fills tiny gaps between cement particles and reacts with calcium hydroxide to form additional cementitious material. This leads to much denser concrete with higher strength and lower permeability. Silica fume is commonly used in bridges, marine structures, industrial floors, and high-rise buildings where durability and long service life are required.
Sources of Silica in Concrete
Silica in Sand as Primary Fine Aggregate
Sand is the most common source of silica in concrete. It is mainly made of quartz, which is a form of silicon dioxide (SiO₂). This sand is used as fine aggregate and makes up a large part of the concrete mix. It helps fill gaps between larger particles and gives concrete its basic shape and volume. The silica in sand also helps improve strength by forming strong bonds during cement hydration. Because of its natural availability and stability, silica-rich sand is used in almost all types of construction work.
Silica from Crushed Stone and Coarse Aggregates
Crushed stone and coarse aggregates also contain a good amount of silica, depending on the rock type used. Granite, quartzite, and sandstone are common sources. These materials provide bulk strength to concrete and improve load-bearing capacity. The silica content in these aggregates helps create a strong internal structure when mixed with cement. However, the type of rock used is important because some silica-rich aggregates can react with cement under certain conditions, affecting long-term durability.
Silica Present in Cement Raw Materials
Cement itself contains silica as one of its key raw materials. During cement production, limestone is combined with materials like clay and shale, which naturally contain silica. This silica plays a role in forming important compounds during hydration, especially calcium silicate hydrate. These compounds are responsible for the hardness and strength of concrete. Without silica in cement raw materials, the final product would not achieve required strength levels.
Silica Fume as a High-Reactivity Additive
Silica fume is an ultra-fine industrial by-product added to concrete to improve performance. It contains a very high percentage of amorphous silica and reacts quickly with cement compounds. When added in small amounts, it fills microscopic voids and makes the concrete much denser. This improves strength, reduces permeability, and increases resistance to chemicals and water. It is widely used in high-performance and durable concrete structures.
Fly Ash and Other Pozzolanic Silica Sources
Fly ash and similar pozzolanic materials are additional sources of silica in concrete. Fly ash is produced from coal combustion and contains reactive silica and alumina. When mixed with cement, it reacts with calcium hydroxide and forms additional binding compounds. This improves long-term strength and reduces heat generation in large concrete structures. Other industrial by-products also contribute silica and help make concrete more sustainable and durable.
Functions of Silica in Concrete
Silica Improves Concrete Strength and Load Capacity
Silica increases the overall strength of concrete by helping form a dense internal structure. During cement hydration, silica reacts and creates calcium silicate hydrate, which is the main strength-giving compound in concrete. This leads to higher compressive strength and better load-bearing capacity. As a result, concrete can handle heavy pressure without cracking or breaking easily.
Silica Reduces Water Permeability in Concrete
One important function of silica is reducing the movement of water inside concrete. It fills small voids and makes the structure more compact. This lowers the number of tiny channels where water can pass through. With less water penetration, concrete becomes more stable and less likely to suffer from internal damage over time.
Silica Enhances Long-Term Durability of Structures
Silica improves the durability of concrete by making it more resistant to environmental effects. Concrete with proper silica content can better withstand weather changes, moisture exposure, and aging. This helps structures last longer with fewer repairs and maintenance needs.
Silica Increases Resistance to Chemical Attack
Silica helps concrete resist harmful chemicals found in soil, water, and industrial environments. It reduces porosity, which limits the entry of aggressive substances like chlorides and sulfates. This protection is especially important for structures like bridges, marine works, and industrial floors.
Silica Improves Abrasion and Surface Wear Resistance
Concrete surfaces often face wear due to traffic, friction, and movement of heavy loads. Silica makes the surface harder and more resistant to abrasion. This is useful for roads, pavements, and factory floors where constant movement occurs.
Silica Strengthens Bond Between Cement and Aggregates
Silica improves the bonding between cement paste and aggregates. A stronger bond reduces weak points inside concrete and increases overall stability. This results in a more uniform and reliable structure that performs better under stress and long-term use.
Silica Fume in Concrete
Fine Composition of Silica Fume Powder
Silica fume is an ultra-fine material made mostly of amorphous silica. It is produced as a by-product during the manufacturing of silicon metal or ferrosilicon alloys. The particles are extremely small, about 100 times finer than cement. Because of this very fine size, silica fume easily fills microscopic gaps between cement particles, making the concrete structure much denser and more compact.
Physical and Chemical Properties of Silica Fume
Silica fume has a very high surface area and strong chemical reactivity. It is light gray to dark gray in color and has a powder-like texture. Due to its amorphous nature, it reacts quickly with calcium hydroxide present in cement. This reaction forms additional calcium silicate hydrate, which improves the internal structure of concrete and increases its strength.
Benefits of Silica Fume in Concrete Performance
Silica fume significantly improves concrete strength and durability. It increases compressive strength, reduces permeability, and enhances resistance to chemicals and water. It also improves bonding between cement paste and aggregates. These improvements make concrete more suitable for demanding environments where high performance is required, such as heavy-load structures and exposed conditions.
Limitations of Using Silica Fume in Concrete Mixes
Despite its benefits, silica fume has some limitations. It can reduce workability of fresh concrete because of its very fine particles. This often requires the use of water-reducing admixtures. It is also more expensive compared to regular cement materials. Improper use or excess quantity can make mixing and placement more difficult.
Common Uses of Silica Fume Concrete in Construction
Silica fume concrete is widely used in structures that require high strength and durability. It is commonly used in bridges, dams, marine structures, tunnels, and high-rise buildings. It is also used in industrial floors and repair works where resistance to wear, chemicals, and water is important for long service life.
Benefits of Silica in Concrete
Higher Compressive Strength for Strong Structures
Silica increases the compressive strength of concrete by forming a dense internal structure. It reacts with cement compounds and creates strong binding materials that hold the mix tightly together. This allows concrete to carry heavier loads without cracking or breaking. Higher strength is especially important in structures like bridges, columns, and high-rise buildings.
Better Long-Term Durability of Concrete
Concrete with silica lasts longer because it is more stable and less porous. It reduces internal gaps where water and chemicals can enter. This makes the structure more resistant to aging and damage over time. As a result, the concrete maintains its quality for many years even under continuous use.
Lower Maintenance Requirements Over Time
Because silica improves durability and strength, concrete structures require less repair work. Cracks, surface damage, and internal weakening happen less frequently. This reduces maintenance costs and makes construction more economical in the long run. It is especially useful for large infrastructure projects where repair work is difficult and expensive.
Improved Resistance to Moisture and Water Damage
Silica makes concrete less permeable, which means water cannot easily pass through it. This reduces problems like dampness, leakage, and internal corrosion of steel reinforcement. Better water resistance is important for structures exposed to rain, groundwater, or marine environments.
Better Performance in Harsh Environmental Conditions
Concrete containing silica performs well in extreme conditions such as high heat, freezing temperatures, and chemical exposure. It resists damage caused by salts, acids, and other aggressive substances. This makes it suitable for industrial areas, coastal regions, and heavy traffic zones.
Longer Service Life of Concrete Structures
All these improvements together increase the overall life of concrete structures. Stronger bonding, reduced permeability, and better resistance to damage help structures remain safe and functional for a longer time. This makes silica an important material for long-lasting construction.
Alkali-Silica Reaction (ASR) in Concrete
What Alkali-Silica Reaction Means in Concrete?
Alkali-Silica Reaction (ASR) is a chemical reaction that happens inside concrete. It occurs when reactive silica in aggregates reacts with alkalis present in cement and water. This reaction produces a gel-like substance inside the concrete. The gel absorbs moisture and starts to expand, which creates internal pressure within the structure.
Main Causes of Alkali-Silica Reaction in Structures
ASR mainly happens due to the presence of reactive silica in aggregates such as certain types of sand and gravel. High alkali content in cement also increases the risk. Moisture plays an important role because the reaction needs water to continue. Poor material selection and uncontrolled mix design can also increase the chances of ASR in concrete structures.
How Alkali-Silica Reaction Damages Concrete?
When the gel formed by ASR absorbs water, it expands inside the concrete. This expansion creates pressure within the hardened structure. Over time, this pressure leads to internal cracking. These cracks slowly spread through the concrete, weakening its internal structure and reducing its overall strength and stability.
Common Signs of Alkali-Silica Reaction Damage
ASR damage can often be seen as fine cracks on the surface of concrete. These cracks usually form in a map-like or irregular pattern. In some cases, the surface may show expansion, uneven edges, or small gel deposits. The concrete may also start to lose its smooth finish and appear rough or damaged.
Effects of ASR on Structural Performance
ASR weakens the internal structure of concrete over time. It reduces strength, durability, and load-bearing capacity. As cracks grow, they allow more water and harmful substances to enter the concrete, causing further damage. If not controlled, ASR can shorten the service life of structures and increase maintenance needs significantly.
How to Prevent Alkali-Silica Reaction in Concrete
Use Non-Reactive Aggregates for Safe Concrete
One of the most effective ways to prevent Alkali-Silica Reaction is to use non-reactive aggregates. These are materials that do not contain reactive forms of silica. Before using sand or stone in concrete, they are tested for reactivity. Selecting safe aggregates reduces the chance of harmful chemical reactions inside the concrete structure.
Use Low-Alkali Cement to Reduce Chemical Risk
Cement with low alkali content helps reduce the risk of ASR. Alkalis in cement are a key factor in triggering the reaction with silica. By using low-alkali cement, the overall chemical activity inside the concrete is reduced. This helps in controlling unwanted expansion and cracking over time.
Add Pozzolanic Materials for Better Stability
Pozzolanic materials like fly ash, silica fume, and slag are commonly used to control ASR. These materials react with calcium hydroxide in cement and reduce the amount of free alkalis. This improves the internal structure of concrete and makes it less reactive. It also increases strength and long-term durability.
Control Moisture Exposure in Concrete Structures
Moisture is necessary for Alkali-Silica Reaction to occur. Without water, the reaction slows down or stops. Proper drainage, waterproofing, and protective coatings help reduce water entry into concrete. Keeping structures dry is an important step in controlling ASR damage.
Proper Concrete Mix Design for Long-Term Safety
A well-planned concrete mix plays a major role in preventing ASR. The right balance of cement, aggregates, water, and additives reduces the chances of harmful reactions. Engineers carefully design the mix based on material properties and environmental conditions. A controlled mix ensures better stability and longer service life of concrete structures.
Silica Content in Different Types of Concrete
Silica in Standard Concrete for Basic Construction
Standard concrete contains silica mainly from natural sand and gravel. These materials provide the basic strength and structure of concrete. The silica in this type of concrete helps form calcium silicate hydrate during cement hydration, which gives hardness and stability. Standard concrete usually has a balanced silica content that supports general construction work like floors, pavements, and small buildings where extreme strength is not required.
Silica in High-Strength Concrete for Heavy Loads
High-strength concrete contains a higher and more controlled silica content. It often includes fine silica materials like silica fume to improve density and reduce internal voids. This results in much higher compressive strength and better load-bearing capacity. The reduced porosity also improves durability, making it suitable for bridges, skyscrapers, and heavy infrastructure projects. In this type, silica plays a key role in achieving a compact and strong internal structure.
Silica in Self-Compacting Concrete for Easy Flow
Self-compacting concrete has a carefully balanced silica content to maintain flowability without losing strength. Silica-rich fine particles help improve cohesion and prevent separation of materials while the concrete flows under its own weight. This makes it easy to fill complex shapes and heavily reinforced structures without vibration. It is widely used in modern construction where speed and smooth finishing are important.
Silica in Precast Concrete for Controlled Production
Precast concrete uses controlled silica content to ensure consistent quality. Since these elements are cast in factories, the mix design includes carefully selected silica sources like sand, fly ash, or silica fume. This improves early strength and ensures smooth finishing, making it suitable for beams, panels, pipes, and structural components produced off-site. The controlled environment allows better optimization of silica content for uniform performance.
Silica in Marine Concrete for Harsh Environments
Marine concrete contains higher silica content with additives like silica fume to improve resistance against saltwater and chemical attack. The dense structure created by silica reduces water penetration and protects steel reinforcement from corrosion. This makes it ideal for ports, docks, bridges, and coastal structures where exposure to moisture and chlorides is constant.
Applications of Silica in Concrete
Silica in Bridges and Highways for Heavy Load Support
Silica is widely used in bridges and highways because these structures carry continuous heavy traffic. It improves strength, reduces cracking, and increases resistance to wear caused by vehicles. The dense structure created by silica also helps protect steel reinforcement from corrosion, which is important for long-term safety in transportation infrastructure.
Silica in Industrial Floors for High Wear Resistance
Industrial floors face constant movement of machinery, heavy loads, and friction. Silica improves surface hardness and reduces abrasion damage. It also lowers dust formation and increases durability. This makes silica-rich concrete suitable for factories, warehouses, and production units where strong and long-lasting flooring is required.
Silica in Dams and Water Retaining Structures
Dams, canals, and water tanks need concrete that can resist continuous water pressure. Silica reduces permeability and helps block water movement through concrete. It also improves resistance to chemical reactions caused by minerals in water. This ensures better safety and long service life of water-retaining structures.
Silica in Marine Structures for Saltwater Protection
Marine structures like ports, docks, and coastal bridges are exposed to saltwater and harsh conditions. Silica makes concrete more compact and less porous, which reduces chloride penetration. This helps prevent corrosion of steel reinforcement and increases durability in marine environments where damage risk is high.
Silica in High-Rise Buildings for Strength and Stability
High-rise buildings require very strong and stable concrete. Silica increases compressive strength and improves load-bearing capacity. It also enhances bonding between materials, which helps maintain structural stability under vertical loads. This makes it an important material in modern tall building construction.
Silica in Tunnels and Large Infrastructure Projects
Tunnels and underground structures need concrete that can resist pressure, moisture, and chemical exposure. Silica improves density and reduces water leakage through cracks. It also increases durability under continuous stress. This makes it suitable for metro systems, underground passages, and other major infrastructure works.
Safety Concerns of Silica in Concrete
Respirable Crystalline Silica and Fine Dust Particles
Respirable crystalline silica is a very fine dust created when materials like sand, stone, or concrete are cut, drilled, or crushed. These tiny particles are invisible to the eye and can stay in the air for a long time. When inhaled, they can reach deep into the lungs. This type of silica is mainly a concern during construction and demolition work, not in hardened concrete.
Health Risks Linked to Silica Dust Exposure
Long-term exposure to silica dust can cause serious health problems. It can lead to lung diseases such as silicosis, chronic bronchitis, and even increase the risk of lung cancer. Breathing in fine silica dust damages lung tissue over time, reducing breathing capacity. Short-term exposure may cause coughing, irritation, and breathing discomfort, especially in poorly ventilated areas.
Construction and Industrial Work at Higher Risk
Workers in construction, mining, stone cutting, and concrete manufacturing face higher exposure to silica dust. Activities like cutting concrete, drilling, grinding, and demolition release large amounts of fine dust. These jobs require strict safety measures because exposure levels can be high during daily operations.
Safety Measures to Control Silica Exposure
Controlling dust at the source is the most effective way to reduce risk. Wet cutting methods, water sprays, and proper ventilation help limit dust in the air. Work areas should be cleaned regularly to prevent dust buildup. Using enclosed machines and dust extraction systems also helps reduce exposure during concrete work.
Personal Protective Equipment for Worker Safety
Personal protective equipment is essential in environments with silica dust. Workers should use proper masks or respirators designed to filter fine particles. Protective clothing, goggles, and gloves also help reduce contact with dust. Regular training and safety awareness ensure workers understand the risks and follow correct safety practices during concrete-related activities.
Silica vs Silica Fume in Concrete
Main Difference Between Silica and Silica Fume
Silica is a natural material found in sand, stone, and aggregates used in concrete. It mainly provides basic strength and structure. Silica fume, on the other hand, is a very fine industrial by-product made during silicon metal production. It is much more reactive and is mainly used to improve concrete performance, not just as a filler.
Composition Difference Between Both Materials
Normal silica used in concrete is mostly crystalline silica found in natural materials like quartz sand. It has a stable structure and lower reactivity. Silica fume is mostly amorphous silica with extremely fine particles. It is more than 100 times smaller than cement particles and has a very high surface area, which makes it highly reactive in cement mixtures.
Performance Difference in Concrete Applications
Silica mainly contributes to the basic strength and stability of concrete through aggregates. It forms the skeleton of the structure. Silica fume works differently. It reacts with cement compounds and forms extra binding material, which increases strength, reduces permeability, and improves durability. It also makes concrete more resistant to water, chemicals, and corrosion.
Application Difference in Construction Work
Silica is used in almost all types of concrete as part of sand and aggregates, making it essential for standard construction. Silica fume is used only in special cases where high performance is needed. It is commonly used in bridges, marine structures, tunnels, high-rise buildings, and industrial floors where extra strength and long life are important.
Factors Affecting Silica Performance in Concrete
Particle Size of Silica Materials in Concrete
Particle size plays an important role in how silica performs in concrete. Fine silica particles, like silica fume, fill very small gaps between cement grains and make the structure denser. Coarser silica, found in sand and aggregates, mainly provides basic strength and structure. Smaller particles react more quickly with cement and improve bonding, while larger particles mainly act as structural support. The balance between fine and coarse silica affects overall concrete quality.
Water-Cement Ratio and Its Effect on Silica Action
The water-cement ratio is one of the most important factors in concrete performance. When too much water is used, concrete becomes porous and weak, reducing the effect of silica. A lower water-cement ratio produces a denser mix where silica can work more effectively to improve strength and durability. A well-controlled ratio helps silica form stronger bonds and reduces internal voids in concrete.
Cement Composition and Its Reaction with Silica
The type and composition of cement directly affect how silica performs. Cement contains compounds that react with silica to form calcium silicate hydrate, which is the main strength-giving material in concrete. If cement has higher alkali content, it can also increase the risk of unwanted reactions like ASR. Proper cement selection ensures that silica contributes positively to strength and durability without causing damage.
Curing Conditions and Their Role in Silica Effectiveness
Curing conditions control how well cement and silica react over time. Concrete needs proper moisture and temperature to allow full hydration. If curing is poor or the surface dries too quickly, silica cannot fully react with cement, reducing strength and durability. Good curing ensures continuous chemical reactions that improve the internal structure of concrete.
Aggregate Quality and Its Impact on Silica Performance
The quality of aggregates also affects silica performance in concrete. Strong, clean, and durable aggregates allow better bonding with cement paste and silica compounds. Poor-quality aggregates with impurities or weak structure can reduce overall strength. Well-graded aggregates help silica fill voids more effectively, resulting in a denser and more stable concrete structure.
Advantages and Limitations of Silica in Concrete
Advantages of Using Silica in Concrete for Better Performance
Silica improves the overall performance of concrete in many important ways. It increases compressive strength by forming strong bonding compounds inside the cement matrix. This helps concrete handle heavy loads without breaking easily. Silica also improves durability, making structures last longer even under continuous use and exposure.
Another major benefit is reduced permeability. Silica fills tiny gaps in the concrete mix, which limits water and air movement inside the structure. This helps protect steel reinforcement from corrosion and reduces internal damage over time. It also improves resistance to chemicals such as chlorides and sulfates, which are commonly found in soil, seawater, and industrial environments. Because of these properties, silica is widely used in infrastructure that requires long-term strength and stability.
Limitations of Silica in Concrete and Practical Challenges
Despite its advantages, silica also has some limitations. One major concern is the risk of Alkali-Silica Reaction (ASR), where reactive silica in aggregates reacts with cement alkalis and causes internal expansion and cracking. This can damage concrete if proper materials are not selected.
In some cases, using silica-rich materials or additives can increase overall construction cost. High-quality silica materials like silica fume are more expensive than normal cement components, which can affect budget planning in large projects.
Silica fume can also create workability challenges. Because of its very fine particles and high surface area, it makes the concrete mix sticky and harder to handle. This often requires additional water-reducing agents to maintain proper flow and placement.
Another important limitation is health risk during handling. Fine silica dust generated during cutting or grinding can be harmful if inhaled. Proper safety measures are required at construction sites to reduce exposure and protect workers.
Overall, silica provides strong benefits, but it must be used carefully to avoid these limitations.
Conclusion
Silica is one of the most important materials used in concrete. It is naturally present in sand and aggregates and also used in advanced forms like silica fume. It plays a key role in improving the strength, durability, and overall performance of concrete structures. By helping form strong binding compounds and reducing internal gaps, silica makes concrete denser and more reliable.
At the same time, silica must be used in a controlled way. If reactive silica is not managed properly, it can lead to problems like Alkali-Silica Reaction, which can damage concrete over time. This is why proper material selection, mix design, and quality control are very important in construction work.
In modern construction, silica is widely used in roads, bridges, buildings, marine structures, and industrial projects. Its ability to improve long-term performance makes it a key part of both standard and high-performance concrete.
Overall, silica is essential for creating strong and durable concrete, but it must be handled with care to ensure safety and long-lasting results.
Frequently Asked Questions (FAQs)
What does silica do in concrete?
Silica helps concrete gain strength and stability. It reacts with cement and forms strong binding compounds that make concrete hard and durable.
Is silica good for concrete?
Yes, silica is good for concrete when used in the right amount. It improves strength, reduces water movement, and increases durability.
What is silica fume in concrete?
Silica fume is a very fine powder made during metal production. It is added to concrete to improve strength, reduce pores, and increase durability.
What causes alkali-silica reaction?
ASR happens when reactive silica in aggregates reacts with alkalis in cement in the presence of moisture, forming an expanding gel inside concrete.
How can ASR be prevented?
ASR can be prevented by using non-reactive aggregates, low-alkali cement, and adding materials like fly ash or silica fume.
Is silica sand used in concrete?
Yes, silica sand is commonly used as a fine aggregate in concrete. It provides basic strength and helps form the structure of concrete.
What are the benefits of silica fume?
Silica fume increases concrete strength, reduces permeability, improves durability, and enhances resistance to water and chemicals.
Is silica harmful during concrete work?
Silica dust can be harmful if inhaled for a long time. Proper safety gear and dust control measures are needed during cutting or grinding work.
Which concrete applications use silica fume?
Silica fume is used in bridges, tunnels, marine structures, industrial floors, and high-rise buildings where high strength and durability are needed.
What is the difference between silica and silica fume?
Silica is a natural material found in sand and stone, while silica fume is an ultra-fine industrial by-product used to improve concrete performance.